Lavinthal discards are suit-preference signals that work in the following manner: the first
discard by either defender denies interest in that suit, and the size of the discard
indicates whether he would like the higher or lower of the two remaining suits led.

For example, if declarer is running diamonds, a high heart discard would ask for a
spade, and a low heart discard would ask for a club. Similarly, a high spade discard would
ask for a heart, and a low spade discard would ask for a club.

The main advantage is that the defender does not need to part with what might be a
valuable card in the suit he actually wants. He also has a choice of which of the
remaining two suits to use for his signal.

The disadvantage of course is that one of the defenders may forget that the partnership
is playing this signalling convention!